An organic EL element which is an example of an organic optical device has been actively researched and developed. In the fundamental structure of the organic EL element, a light-emitting organic compound is interposed between a pair of electrodes (an upper electrode and a lower electrode). By applying voltage to this element, light emission from the light-emitting organic compound can be obtained.
The solid-state light-emitting element can be formed into a film; thus, a large-area element can be easily formed. Therefore, the solid-state light-emitting element has a high utility value as a surface light source that can be applied to lighting or the like.
However, as the area of a light-emitting portion in a lighting device or a display device having a large screen increases, a voltage drop due to the resistance of an upper electrode or a lower electrode of a light-emitting element tends to be significant. In the case where a voltage drop in the electrodes is significant, there is a problem in that unevenness in luminance might be seen. In order to solve such a problem, an auxiliary electrode formed using a material having low resistivity needs to be connected to the electrodes.
In Patent Document 1, a structure in which an auxiliary electrode is formed over a transparent electrode is reported. It is much needed to provide an auxiliary electrode for a transparent electrode which is provided on the light extraction side in a light-emitting element. This is because a light-transmitting material used for a transparent electrode has relatively high resistivity. In Patent Document 1, the auxiliary electrode having low resistivity is formed over the transparent electrode to support conductivity of the transparent electrode.